In the wake of last November's election, many Democrats blamed low turnout for Kamala Harris's defeat. It wasn't entirely without reason, as turnout dropped in Democratic areas, but many months later it is clear the blame was misplaced. Newly available data, based on authoritative voter turnout records, suggests that if anything, President Trump would have done even better if everyone had voted. The new data, including a new study from Pew Research released Thursday, instead offers a more dispiriting explanation for Democrats: Young, nonwhite and irregular voters defected by the millions to Mr. Trump, costing Ms. Harris both the Electoral College and the popular vote.